GTUBE

The GTUBE ("Generic Test for Unsolicited Bulk Email") is a 68-byte test string used to test anti-spam systems, in particular those based on SpamAssassin. In SpamAssassin, it carries an antispam score of 1000 by default, which would be sufficient to trigger any installation.

and should be placed in the message body of an RFC 5322 compliant email message, without any linebreaks or whitespaces.

There exist some varieties, notably the NAItube (which will carry a variable weight) and the GTphish (which will trigger specifically as a phishing mail), which are used in the McAfee implementation of SpamAssassin.